Sensors are typically used for detecting environment parameters, such as light intensity, sound, pressure, and the like. Sensors are also widely used in imaging applications, such as infrared imaging for night vision. Existing sensors are often integrated with integrated circuits, for example, application specific integrated circuits (ASIC), which may in-situ process the sensed signals. For imaging applications, a great amount of processing may be involved. Integrating the sensors with the ASIC is thus advantageous for improving the performance.
Conventionally, image sensors that respond to photons were formed at the surface of (or even “in”) semiconductor substrates. FIG. 1 illustrates a cross-sectional view of photo diodes 4 built in semiconductor substrate 2. Photo diodes 4 may be formed as an array. To suit the requirement of the ASIC, which is formed on the same semiconductor substrate 2, a plurality of metallization layers 6 (sometimes up to nine layers) are formed over photo diodes 4. Metallization layers 6 include dielectric layers, and metal lines formed in the dielectric layers. Further, passivation layer(s) 8 are formed on metallization layers 6.
For photo diodes 4 to sense photons, the photons (symbolized by arrows 10) have to penetrate the dielectric layers in metallization layers 6 and the passivation layer(s) 8. This causes the degradation in the signal strength received by photo diodes 4. Further, metallization layers 6 typically include low-k dielectric layers, and etch stop layers (ESLs) between the low-k dielectric layers. The ESL layers and low-k dielectric layers have different refractive indexes, resulting in the reflection and deflection of the photons. As a result, cross-talk occurs. For example, the non-uniformity of the ESLs and the low-k dielectric materials may cause the non-uniformity in the deflection, and hence photons 12, which are destined to photo diode 41 to be received by photo diode 42. The sensed image is thus distorted.
The conventional sensor formation has conflicting requirements with the formation of the ASIC. To reduce the adverse effect caused by layers 6 and 8, it is preferred that inter-layer dielectric (ILD) and inter-metal dielectrics (IMD) are as thin as possible. However, reducing the thicknesses of ILD and IMDs causes process difficulty and possible performance degradation for the ASIC, and may require customized formation processes. New methods for forming sensors are thus needed for solving the above-discussed problems.